Sunday, November 6, 2011

Creating PCBs - Exposing the board

Now it's time to expose the photosensitive film to the UV light through the negative artwork mask. To get a clear image it's important that the artwork be held tightly against the board.
For this, I've used a cheap $2 plastic photo frame that I've cut down the length to fit neatly into the UV lamp. In this is (from the top)

  1. Glass (which I broke the corner off, cutting it down to fit the re-sized photo frame. OK, I suck at cutting glass)
  2. 1 sheet of rapidraw film, just to diffuse the UV light a little
  3. The artwork
  4. The copper board with photosensitive film
  5. A sheet of A4 paper, folded, to give the sandwich some more thickness
  6. The cut-down original backing plate of the photo frame.
It looks like this.

So this assembly goes into the UV lamp, like so:
And is exposed.

Exposure timing
I'd previously done an exposure timing test with the film, but not with the film on copper board. The exposure test is done by progressively covering sections of the film, and determining which section has been exposed sufficiently where the artwork is clear, but not exposed where the artwork is dark.
Too little exposure, and the film won't harden enough (with negative film, the exposed areas of film harden)
Too much, and the areas that shouldn't have been exposed may start to harden. This is why the artwork contrast is very important.
OK, so prior experimentation indicated that about 2 minutes should be enough. With this test board, I had three identical patterns, so I exposed one section for 2 minutes, the next for 2:30, and the last for 3 minutes.

This is the result:
From left to right, 3 min, 2.5 min and 2 min exposure
The vendor says that the film will darken where it is exposed, and you can see that is the case. In fact, at this point, it all looks quite good. Now to remove the un-exposed areas with the developing solution.
3 minutes on left, 2 minutes on right
Hmm, this shows some problems. The rightmost numerals have etched away in some cases. I'm not sure if it's due to underexposure, or if the film was not bonded sufficiently with the copper. The center top, I don't know WTF happened there - the only explanation I can come up with was that the artwork wasn't pressed well enough against the film, and some light has spilled? I would have expected the actual artwork to be blurry if that were the case, however. The leftmost exposure looks good.
The actual development of the film occurs pretty quickly (it's more of a "stripping" of the un-exposed regions), and I'm not sure yet what occurs if the film is over developed.

Next time, we'll see what the copper etching reveals...

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